To rig a sand trap as a rappelling anchor, do the following:
Locate good geometry. Good geometry usually involves some sort of vertical lip or ledge that can keep the sand trap from sliding. It also includes a place for the sand to exit out the sides when done.
Fill the inside of the sand trap with sand. The outside is the side with the pull strap.
Attach your rappel rope to the two handles.
You may want to leave an extra long tail that can be attached to a backup meat anchor.
Alternatively, you could have a person stand near the sand trap, ready to jump on top if it starts sliding.
Send the first rappeller down to test the sand trap. The sand trap may lift or slide slightly - this is normal. The meat anchor should leave the rope loose unless the sand trap slides more than a foot or two.
Save your lightest, smoothest rappeller for last. If the sand trap was strong enough for the big guy, it will be strong enough for the little guy too.
Send all your people down except one.
The last person connects a second rope to the pull strap and deploys the pull rope to the bottom.
The last person rappels very smoothly, employing soft starts and sliding on their hip when possible.
Once everyone is down, pull on the pull rope. This squeezes the sand trap in the middle and pushes the sand out the sides. It may take a couple people to pull on the rope until the sand trap comes down.
Gotchas:
Not enough sand to support the weight of the rappeller. This won't kill you unless you're the last one down. Add more sand and test with the next person. Consider 'tostada method' below.
Sand can't exit the sides, and you can't retrieve your sand trap after rappelling. You could try ascending the rope, but this is very dangerous and the sand trap could dislodge at any moment. You will probably have to cut the rope and leave the sand trap behind.
Sand trap is backwards. This will make the pull harder but probably will still work.
Sand trap gets stuck in a chockstone or tree or other obstruction on the way down. You will probably have to cut the rope and leave the sand trap behind.
Bad geometry. Sand trap may not be appropriate. Consider other anchor options.
Taco Method vs. Tostada Method: The traditional sand trap setup involves folding the the sand trap in half like a hard-shelled taco. This is the taco method. Another option is the tostada method which involves laying the sand trap flat open and piling sand on top of it. The rope then only attaches to one of the handles. The taco method is good when there is a lip or something that goes vertical. The tostada method is better when the sand trap is on a flat ledge. The tostada method can hold more sand and therefore more weight, but it doesn't work well when the ground is sloped.
Advantages
The sand trap is fast and easy to inspect. It can be tested and backed up before the last person rappels. It works when there is sand and good geometry.
Disadvantages
The sand trap does not work in water or when there isn't sand to mine. It doesn't work on downward sloping surfaces. The sand trap is a marginal anchor, meaning it is not very strong. It can also get stuck, resulting in you needing to cut the rope and lose your sand trap.
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